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Survival and Recovery of Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes on Lettuce and Parsley as Affected by Method of Inoculation, Time between Inoculation and Analysis, and Treatment with Chlorinated Water

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The effects of method for applying inoculum and of drying time after inoculation on survival and recovery of foodborne pathogens on iceberg lettuce and parsley were studied. Five-strain mixtures of Escherichia coli O157:H7, Salmonella, or Listeria monocytogenes were applied to lettuce and parsley by dip, spot, or spray inoculation methods. Inocula were dried for 2 h at 22°C or for 2 h at 22°C and then 22 h at 4°C before being treated with water (control) or chlorine (200 μg/ml). Significantly higher populations (CFU per lettuce or parsley sample) of E. coli O157:H7 and Salmonella (α = 0.05) were recovered from dip-inoculated produce than from spot- or spray-inoculated produce. This difference was attributed to larger numbers of cells adhering to lettuce and parsley subjected to dip inoculation. Populations of E. coli O157:H7 and Salmonella recovered from lettuce inoculated by spot and spray methods were not significantly different, but populations recovered from spot-inoculated parsley were significantly higher than those recovered from spray-inoculated parsley, even though the number of cells applied was the same. Significantly different numbers of L. monocytogenes were recovered from inoculated lettuce (dip > spray > spot); populations recovered from dip-inoculated parsley were significantly higher than those recovered from spot- or spray-inoculated parsley, which were not significantly different from each other. Populations of pathogens recovered from lettuce and parsley after drying inoculum for 2 h at 22°C were significantly higher than or equal to populations recovered after drying for 2 h at 22°C and then for 22 h at 4°C. Significant differences (water > chlorine) were observed in populations of all pathogens recovered from treated lettuce and parsley, regardless of inoculation method and drying time. It is recommended that spot inoculation with a drying time of 2 h at 22°C followed by 22 h at 4°C be used to determine the efficacy of chlorine and other sanitizers in killing foodborne pathogens on lettuce and parsley.


Document Type: Research Article

Affiliations: 1: Center for Food Safety and Department of Food Science and Technology, University of Georgia, 1109 Experiment Street, Griffin, Georgia 30223-1797 2: Department of Food Science and Technology, One Shields Avenue, University of California, Davis, California 95616-8598, USA

Publication date: June 1, 2004

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